Tumor suppressor genes are a class of genes identified on the basis of an association between neoplasia and the loss of function in both copies of the gene. Of the 15 to 20 tumor suppressor genes identified to date, retinoblastoma, p53, and Wilms' tumor have been investigated to the greatest extent.
Prohibitin, an evolutionarily conserved gene that possesses antiproliferative activity, is postulated to be a tumor-suppressor gene whose expression, when lost, contributes to the immortalization of cells from one or more of the four complementation groups proposed by Pereira-Smith. Jupe et al., Exp Cell Res 218:577-580 (1995). Its sequence, however, is unrelated to that of any previously cloned tumor suppressor gene. Nuell et al., Mol Cell Biol 11:1372-1381 (1991).
Because of the intracellular, antiproliferative activity of its gene product and its tumor-suppression potential, the prohibitin gene has been widely studied. A rat prohibitin has been described by McClung et al. McClung et al., Biochem Biophys Res Comm 164:1316-1322 (1989). The first full length prohibitin cDNA was isolated on the basis of a higher expression of prohibitin mRNA in non-dividing rat liver cells than in regenerating rat liver cells. It was subsequently reported that prohibitin mRNA microinjected into a cell blocked DNA synthesis and that microinjection of an antisense oligonucleotide stimulated cells to divide. The full length sequence of the rat prohibitin cDNA was described by Nuell et al. Nuell et al., Mol Cell Biol 11:1372-1381 (1991).
The human prohibitin gene was localized to chromosome 17q21 using both mouse-human hybrid cell line mapping and in situ hybridization to human metaphase chromosomes. White et al., Genomics 11:228-230 (1991). This region is very near the location of the familial breast cancer (BRCA 1) locus, and prohibitin was initially considered a candidate gene (Black, D. and Solomon, E., Trends in Genetics 9:22-26 (1993)); however, more recent genetic and cytogenetic mapping studies have ruled out prohibitin as the BRCA 1 gene. Black, et al., Am J Hum Genet 52:702-710 (1993).
At the mRNA level, it has been reported that the human prohibitin gene is expressed as two transcripts of 1.2 kb and 1.9 kb in normal human cells. Liu, et al., Biochem Biophys Res Comm 201:409-414 (1994). The difference between the two transcripts is an approximately 750 nucleotide sequence 3′ to the end of the 1.2 kb transcript. Despite these differences, both code for the identical protein. Analyses have shown that the 1.9 kb transcript is expressed to a greater extent during the G1/S and S phases of the cell cycle in human populations (Liu et al., Biochem Biophys Res Comm 201:409-414 (1994)) and that this transcript is overexpressed in immortalized cell populations derived from human tumors. Jupe et al., Exp Cell Res 218:577-580 (1995).
Specifically, prohibitin is apparently involved in the process of immortalization in a group of human cells that are classified in complementation Group B proposed by Olivia Pereira-Smith. Cells classified in this group exhibit a unique prohibitin genotype. Upon Southern analysis of EcoRI digested DNA using a probe to intron 4 of prohibitin, DNA from Group B cells exhibit only one band at 5 kb. Normal DNA analyzed in the same way exhibits one of three banding patterns: two bands, one at 5 and one at 7 kb; one band at 5 kb; or one band at 7 kb. White et al., Genomics 11:228-230 (1991); Tokino et al., Internatl J Onco 3:769-772 (1993); Jupe et al., Exp Cell Res 218:577-580 (1995). DNA from cells classified in the other groups (A, C, D) exhibit one of two banding patterns, either one 7 kb band, or one 7 kb and one 5 kb band with the 7 kb band yielding a stronger signal; however, no sample from these complementation groups exhibit only the 5 kb band. These results indicate that prohibitin exists as two alleles, one of which contains an additional EcoRI cut site and is characterized by a 5 kb band on Southern analysis. Since only immortalized cells classified as Group B are uniformly homozygous for the allele yielding the 5 kb band, this allele has been designated the B type allele and the other allele yielding the 7 kb band has been designated the non-B type allele. Thus, it appears that the loss of heterozygosity plays a role in cellular immortalization of Group B cells.
A human prohibitin cDNA, including 134 nucleotides immediately 3′ to the stop codon of the coding sequence, has been reported. In addition, mutations occurring in exon 4 and exon 5 of this sequence were found in material derived from four of twenty-three sporadic human breast tumors. Sato et al., Cancer Res 52:1643-1646 (1992). A study of the four most highly conserved exons of the prohibitin gene (exons 4 through 7) from DNA purified from blood samples from seventy-six familial breast cancer patients found no alterations in the prohibitin coding region, but they did find two additional intronic polymorphisms, one in intron 4 and one in intron 5. However, there was no evidence that any of the patients carried a germline change of the conserved prohibitin gene region. It was concluded that mutations in the prohibitin gene were not associated with the early onset, familial form of the disease. Tokino et al., Internatl J Oncol 3:769-772 (1993).
In a more extensive study, an RNase protection assay was used to screen 120 primary breast tumors that showed a loss of heterozygosity for the long arm of chromosome 17 and/or were derived from patients younger than age thirty-five. This assay was also used to test for prohibitin alterations in a number of ovarian, liver, and lung tumors. One additional mutation in one of the breast tumors and no mutations in any of the other samples were found. The investigators concluded that somatic mutations in prohibitin may be associated with sporadic breast tumors, but may not be a factor in a large number of other tumors, including early onset breast cancer. Sato et al., Genomics 17:762-764 (1993). This conclusion was reinforced by two additional studies. Cliby et al. found no mutations in prohibitin exons 4 or 5 from twenty human ovarian tumors (Cliby et al., Gynecologic Oncology 50:34-37 (1993)), and Asamoto and Cohen were unable to find prohibitin mutations in cDNAs from a series of rat bladder tumors and tumor cell lines (Asamoto, M. and Cohen, S. M., Cancer Let 83:201-207 (1994)).
While these studies presented evidence which indicated a lack of any relationship between inherited breast cancer and prohibitin mutations, whether an association existed between prohibitin mutations and sporadic or late onset breast cancer remained an unanswered question. The reported studies into the relationship between prohibitin mutations and sporadic or late onset breast cancer, however, focused on the coding region of the gene and whether its mutation(s) corresponded with the development of cancerous tumors.
It has now been found that mutations in the 3′ untranslated region (UTR) of the B type allele are diagnostic for increased susceptibility to cancer, particularly breast cancer. Thus, the presence (or absence) of these mutations can be used as a screening tool for the early detection and treatment of cancer. In another embodiment, reintroduction of a normal 3′ UTR into early stage tumors can be employed as a therapeutic agent for treatment of cancer.